A. Field of Invention
The present invention is directed generally to a method and system for controlling operation of a lockup clutch provided to a motor vehicle's transmission, and more particularly, to a method and system for the control of a clutch for fluid coupling an engine output and a transmission input to maximize fuel efficiency of the motor vehicle when acceleration of the motor vehicle transitions from one state to another state.
B. Description of the Related Art
Traditionally, an automatic transmission has been provided to motor vehicles to transmit the rotational energy generated by the engine to cause rotation of the tires. The automatic transmission has commonly included a set of planetary gears that selectively cooperate with a sun gear under the control of hydraulically controlled frictional engagement elements to rotate the tires at a speed that causes propulsion of the motor vehicle forward at a desired speed. To allow at least partial independence between the engine's rotation and the rotation of the tires, a torque converter is provided as a fluid coupling between the engine output shaft and the transmission. The torque converter transmits torque from the engine at a gradual, increasing rate to the transmission when a driver of the automobile actuates the acceleration pedal of the motor vehicle. And since the torque converter can allow for somewhat independent operation of the engine and transmission, the torque converter also minimizes the shock experienced by occupants of the motor vehicle during sudden acceleration or deceleration states.
A motor vehicle with an automatic transmission, however, achieves less fuel efficiency than a similar motor vehicle equipped with a manual transmission because the automatic transmission is permitted to “slip” with respect to the engine due to the torque converter. In other words, the torque converter allows the output shaft of the engine to rotate at a different speed than the input shaft of the transmission, thereby eliminating a static, direct one-to-one connection between the output shaft of the engine and the input shaft of the transmission. To overcome this drawback, many automatic transmissions are provided with a lockup clutch within the torque converter so that under predetermined driving conditions, the output shaft of the engine and the input shaft of the transmission are directly connected, causing those shafts to rotate at the same angular velocity. Examples of driving conditions during which the lockup clutch is engaged to directly connect the engine's output shaft to the transmission's input shaft include steady cruising where the motor vehicle is neither accelerating nor decelerating, as well as gradual acceleration and deceleration states.
Under abrupt deceleration conditions, such as when the driver completely releases the accelerator pedal for example, the engine's output shaft is urged to suddenly rotate slower than the transmission's input shaft, since the motor vehicle is traveling at approximately the same velocity it was prior to the deceleration. If the pressure at which the lockup clutch is applied is constant when such deceleration occurs, the lockup clutch will slip, thereby allowing the transmission side of the torque converter to rotate faster than the engine side of the torque converter under those driving conditions. Such slippage will prevent termination of the fuel supply to the engine corresponding to the release of the accelerator by the driver until the two sides of the torque converter are once again returned to approximately equal rotational speeds. Prolonging the delay of the fuel supply termination will consume extra fuel unnecessarily, thereby worsening the fuel efficiency of the motor vehicles with automatic transmissions compared to motor vehicles with manual transmissions.
Attempts have been made to shorten the delay before the fuel supply to the engine is terminated following a deceleration condition. Many of these attempts call for the pressure at which the lockup clutch is engaged to be elevated to a peak pressure until the engine and transmission sides of the torque converter are rotating at the same speed. Doing so, however, causes a sudden and temporary interruption in the momentum of the motor vehicle, an experience commonly referred to as “shock,” which is felt by the occupants of the motor vehicle. The shock experienced by the occupants of the motor vehicle makes for an uncomfortable ride each time a deceleration condition occurs.
Accordingly, there is a need in the art for a method and system for synchronizing the engine and transmission sides of a torque converter provided to a motor vehicle with an automatic transmission. Such a system and method can minimize shock experienced by occupants of the motor vehicle, while minimizing the delay before the supply of fuel to the engine is terminated following the onset of the deceleration condition.